Study On Dynamic Characteristics And Stress Intensity Factors Of Cracked Blades Of Axial Compressor

As an energy conversion device,axial compressor plays an important role in industrial production.At present,the compression machines are manufactured in pursuit of large scale,energy saving and high reliability.Blades are key components in the compressor and their vibration characteristics and fatigue/fracture behavior is of great importance to early fault predictions and intelligent diagnosis methods.In this paper,one of the sixth stage blades of an axial compressor is investigated.The dynamic characteristics of its cracked stucture is analyzed including the natural frequencies and fracture mechanics characteristics such as stress intensity factor through theoretical analysis and numerical simulation.Step by step,derivations are given and modified from a simple model to a complex model,and the analytical expression of the mode-I stress intensity factor of edge crack in axial flow compressor is eventually established.The major parts of the present thesis are summrized as follows:(1)The sixth stage blade model of axial compressor is bulit up using Solid Works,and its modal is analyzed through ANSYS.The results show that the natural frequency increases with the increasing rotate speed while decreases with the increasing crack length,and there is a coupling effect between the two factors.The location of the crack poses an influence on the natural frequency,with the greatest effect happening to the first-order bending vibration frequency at a distance of 37 mm from the bottom.(2)For a two-dimensional model of a rectangular plate with an edge crack,the mode-I stress intensity factor is calculated using ABAQUS.The comparison between numerical solution and analytical solution shows that the numerical calculation using singular elements has a high accuracy.In addition,the length and number of singular elements have little influence on the numerical results.By means of numerical simulation and polynomial fitting,the analytical expression of the shape factor of the model is modified at a small ratio of the length and width.(3)Based on the weight function method,the semi-analytical solution of the stress intensity factor of edge crack in plate blade is derived under the action of centrifugal force.The size of the plate and the location of the crack are changed,and the semi-analytical solution is compared with the numerical results.The error of the solution is analysed,and the range of applicability of the semi-analytical solution is discussed.The cause of large error in the bottom area of the blade is analyzed.The results show that the increasing shear stress leads to the reduction of the mode-I stress intensity factor.The stress intensity factors along plate thickness are calculated using ABAQUS XFEM,and the semi-analytical solutions are corrected by adding a constant.The analytical expression of the maximum mode-I stress intensity factor of the crack in a three dimensional plate model is obtained.(4)The stress intensity factors of edge crack in the blade are calculated using ABAQUS XFEM,and the results are compared with the modified semi-analytical solution.The mode-I SIFs with different crack lengths and positions are calculated,and the correction parameters are obtained through using polynomial fitting.Finally,the semi-analytical correction formula is provided.Tested by numerical calculation,the formula shows a certain degree of accuracy.